Physics – Condensed Matter – Strongly Correlated Electrons
Scientific paper
2011-09-02
Physics
Condensed Matter
Strongly Correlated Electrons
12 pages, 5 figures
Scientific paper
In magnetic materials frustration arises when the pairwise interactions between the magnetic moments cannot be satisfied simultaneously, leading to exotic properties such as spin liquid or spin ice at low temperatures. Some cases, however, exhibit spin glass behaviors, and it has been theoretically and experimentally challenging to understand whether or not the spin glass state is an intrinsic ground state. Here we have studied a quasi-two dimensional triangular lattice of bi-pyramids with dominant antiferromagnetic nearest neighbor interactions. We show that the magnetic problem can be mapped onto a problem of two independent degrees of freedom, tri-color and binary sign: the color represents the director of the collinear spins of each bi-pyramid, while the binary sign represents spins within each bi-pyramid that are either parallel or antiparallel to the director. At the mean-field level, the tri-color orders long range in a \surd3 \times \surd3 structure, while the binary sign has numerous degenerate ground states. When combined, the two degrees of freedom yield the collinear bi-pyramid spin ground state for the hybrid lattice. The infinite number of coplanar bi-pyramid spin states can be systematically generated by collective rotations of spins from collinear states. For the long range ordered collinear and coplanar states, we identify 'partial but extensive' spin zero-energy modes of excitations that are qualitatively different from the 'local' zero-energy excitations found in the spin liquid states of the pure two-dimensional kagome and pure three-dimensional pyrochlore lattices. We argue that due to the infinite ground state degeneracy and the unique characteristics of the zero-energy excitations, the ground state of the quasi-two-dimensional hybrid antiferromagnet becomes a spin glass.
Cheong Sang-Wook
Iida Kei
Lee Hong Seok
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